The present invention relates to an electrical plug receptacle having multiple sensors for sensing the temperature at multiple points within an enclosure of the receptacle, and that automatically shuts off when any of these temperatures reach a pre-determined temperature.
Many fires are believed to be caused by overloaded electrical outlets, that is, outlets operated with more power transfer than the outlet was designed for. Fires are sometimes caused by a loose connection, a glowing connection and/or a high resistance path. A glowing connection occurs when copper oxide is formed between a copper wire and a screw in a small air gap creating carbon which glows.
The condition of too much power usage is always accompanied by increased temperature in at least one of the ambient temperature, the receptacle temperature and the temperature of a prong of an electrical plug inserted into the receptacle, collectively referred to herein as “operating temperature”. To avoid fires, it is desirable for the outlet to sense when the operating temperature is too hot, and to cease operation.
Bimetallic switches are electromechanical thermal sensors. The bimetallic or bi-metal portion consists of two different metals bonded together such as brass and Invar. Some bimetallic portions consist of three layers sandwiched together. The metals expand at different rates as they warm, causing the element to twist or curve. The changing geometry is used to make or break an electrical contact. Once temperature has returned to normal levels, they revert back to their original geometry.
For a bimetal comprising brass and invar, the bending occurs at a metal temperature of about 200° F.; the actual temperature threshold is determined by the design of the bimetal and its materials. The metal can be heated by a loose connection or by ambient air temperature. Typical plastic household wiring insulation and outlet housing melts at a temperature of about 300° F. but operation above 200° F. is not recommended due to its high probability of material distortion.
It has been proposed to use a bimetal to sense the temperature of an electrical receptacle, and to interrupt operation of the receptacle when the temperature increases beyond a threshold. In practice, proposed designs do not trigger properly because of the time it takes for heat to propagate from the fault area to the bimetal.
Accordingly, there is a need for an electrical receptacle with improved thermal sensitivity.